When there is a prolonged OKR or VOR stimulus (e.g., if you keep turning in one direction), these reflexes will initially counterrotate the eyes in an attempt to maintain a stable image on the retina, as described earlier. However, with a prolonged stimulus, the eyes will reach their mechanical limit, no further compensation will be possible, and the image will begin to slip on the retina. To avoid this situation, a fast saccade-like movement of the eyes occurs in the opposite direction, essentially resetting the eyes to begin viewing the visual scene again. Then the slow OKR-or VOR-induced counterrotation will start anew. This alternation of slow and fast movement in opposite directions is nystagmus and can be recorded on a nystagmogram (
Fig.
9.26 ). Thus nystagmus can be defined as oscillatory or rhythmic movements of the eye in which there is a fast phase and a slow phase. The nystagmus is named according to the direction of the fast phase because the fast phase is more easily observed.
In addition to being induced physiologically by VOR or OKR stimuli, nystagmus can result from damage to the vestibular circuits, either in the periphery (e.g., cranial nerve •Fig. 9.26 Nystagmogram Showing Eye Movements That Occur During Nystagmus. Theplotshowsaleftnystagmusbecausethefastphaseisdirectedtowardtheleft(downwardonthegraph).
VIII) or centrally (e.g., vestibular nuclei), and can be an informative diagnostic symptom.
Conjugate eye movement is movement of both eyes in the same direction and in an equal amount. Such coordination allows a target to be maintained on both foveae during eye movement and is necessary to maintain binocular vision without diplopia (double vision). However, when objects are close (<30 m), maintaining a target on both foveae requires non-identical movements of the two eyes. Such disjunctive, or vergence, movements are also necessary for fixation of both eyes on objects that are approaching or receding. Stimuli that trigger vergence movements are diplopia and blurry images. It should be noted that when tracking an approaching object in addition to convergence movements, the lens accommodates for near vision, and pupillary constriction occurs.
Motor Neurons of the Extraocular Muscles
Three cranial nerve nuclei supply the extraocular muscles: oculomotor, trochlear, and abducens nuclei. These three nuclei are sometimes referred to collectively as the oculomotor nuclei; however, the context (the specific nucleus or all three) should be clear. Motor neurons for the ipsilateral medial and inferior recti, ipsilateral inferior oblique, and contralateral superior rectus muscles reside in the oculomotor nucleus; those for the contralateral superior oblique muscle reside in the trochlear nucleus; and those for the ipsilateral lateral rectus muscle are located in the abducens nucleus. These motor neurons form some of the smallest motor units (1 : 10 nerve-to-muscle ratio), which is consistent with the very fine control needed for precise eye movement.
An important point regarding motor neurons innervating the extraocular muscles is that most have spontaneous activity when the eye is in the primary position (looking straight ahead), and their firing rate is correlated with eye position and velocity. This spontaneous activity allows the antagonist muscle pairs to act in a push-pull manner, which increases the responsiveness of the system. That is, as motor neurons innervating one muscle are activated and cause increased contraction, those innervating its antagonist are inhibited, which leads to relaxation.
In addition to motor neurons, the abducens nuclei have internuclear neurons. These neurons project, via the medial longitudinal fasciculus, to medial rectus motor neurons in the contralateral oculomotor nucleus. As described later, this projection facilitates the coordinated action of the medial and lateral recti muscles that is needed for conjugate movements, such as those that occur in the VOR.
Circuits Underlying the Vestibuloocular Reflex